1. Field of the Invention
This invention relates to an ultrasonic diagnostic apparatus utilizing what is called a color flow mapping (CFM) method, by which the state of bloodstream in an organism is displayed by making use of Doppler effect.
2. Description of the Related Art
An ultrasonic diagnostic apparatus radiates ultrasonic pulses onto an organism, receives ultrasonic echoes reflected from the interface between tissues with different intrinsic acoustic impedance (the product of the density of a medium and the speed of sound), and then processes them to form an ultrasonic tomographic image.
Unlike X-ray diagnostic apparatuses, the ultrasonic diagnostic apparatus eliminates a need for the subject to be exposed to radiation, and allows the tomographic image of soft tissues to appear on the screen without a contrast medium.
Thanks to progress in various technologies including electronic scanning technology, the ultrasonic diagnostic apparatus has been undergoing continuous improvement in the real-time display performance, facilitating the measurement of a moving body. As the apparatus is becoming much easier to operate, its use is spreading widely at a rapid rate.
On top of that, a color flow mapping method, by which the state of bloodstream can be displayed in two dimensions, has recently been developed, which has led to a further expansion of the application field of ultrasonic diagnostic apparatuses.
The color flow mapping method, which makes use of Doppler effect, is a method of producing an image by distributing in a two-dimensional area pixels based on the deviation frequency obtained by comparing the transmitting frequency with the receiving frequency, the image thus produced hereinafter being referred to as the bloodstream image.
Blood vessels stretch intricately between internal organs through the organism. An attempt to two-dimensionally display the state of blood vessels spreading that way is practically impossible. Thus, three-dimensional display techniques, which will be explained below, have been developed and put to practical use.
They include surface display techniques and volume rendering techniques used in the fields of CT (computed tomography) and magnetic resonance imaging, three-dimensional visualization techniques based on stereography making use of human binocular parallax, and rotational display techniques for magnetic resonance angiocardiography.
Any technique described above only offers information on blood vessel tissues, and cannot provide information on bloodstream. Only ultrasonic diagnostic apparatuses are capable of providing bloodstream information. Conventional ultrasonic diagnostic apparatuses, however, offer two-dimensional bloodstream information (a bloodstream image noted above) only. To provide three-dimensional bloodstream information, various methods have been tried: one method is to allocate different colors to individual bloodstream images depending on their depth to synthesize a single image. None of them, however, have come in practice yet because of difficulty in converting three-dimensional representation into two-dimensional one.
Thus, at present, the observer watches bloodstream images of various cross sections on the screen, while moving the probe, and combines them into a three-dimensional representation through his mental work of judging their continuity. This approach may sometimes bring about sufficient three-dimensional bloodstream information for diagnosis. In this method, however, the three-dimensional image is only in his mind, it is impossible to convey the image to others or to record it. In addition to this, there is a serious problem: the difference in ability needed for forming the image, such as an anatomical knowledge, among individuals often lead to the diagnostic result varying from one medical examiner to another.